According to my philosophical understanding there are different kinds of love. Love for the country, love for parents, love for your partner, respect, etc. They are actually really different and I wish one didn't have to club them under one umbrella. If you ask me why love exists, I'd say it is just the effect of natural selection. How it came to be is however a sort of coincidence.

One form of love that took shape which could have had a revolutionary effect on evolution of all present mammals is maternal love. Why do I say so? I'll let you figure that out. (Hint: I define the opposite of this form of love as cannibalism).